Large industrial refrigeration systems conventionally employ ammonia as the refrigerant because of its heat transfer characteristics. Ammonia, however, is a hazardous Group 2 refrigerant. Because of the hazardous nature of ammonia, release of ammonia from pressure vessels in the refrigeration system is strictly regulated under OSHA and EPA regulations.
Ammonia refrigeration systems are pressurized and operate at pressures that can reach 250 pounds per square inch (“PSI”) to 300 PSI. Pressure vessels in ammonia refrigeration systems conventionally have overpressure protection devices to protect the refrigeration systems from damage. The overpressure protection devices may include rupture disc devices, pressure relief valves, or pressure safety valves. When activated, the overpressure protection devices can release the ammonia to the atmosphere, into water, or into some other mitigating fluid or container.
Large industrial ammonia refrigeration systems typically have multiple pressure vessels each with at least one relief valve. The relief valves are located throughout the refrigeration system in order to assure overpressure release from the various pressure vessels in the refrigeration system. When multiple relief valves are used in an ammonia refrigeration system, any release of ammonia refrigerant from any of the relief valves is typically piped into a common collection header, and the released ammonia is either vented to atmosphere outside of the building containing the pressure vessels or piped into a dispersion tank filled with water. Importantly, the common collection header routes the release of ammonia out and away from occupied space.
In a typical industrial ammonia refrigeration system, an ammonia detector commonly referred to as a “sniffer” is located in the common collection header to detect the presence of ammonia in the common collection header and thus detect that a relief valve somewhere in the refrigeration system has opened or is leaking. With the use of a common collection header for multiple relief valves and a sniffer, identifying the location of the pressure vessel experiencing an overpressure condition and the release of ammonia through a particular relief valve becomes virtually impossible because the ammonia quickly fills the entire common collection header before the sniffer detects the presence of ammonia in the common collection header. If the overpressure condition has subsided and the relief valve has reseated, pinpointing the location of the relief valve that released the ammonia or the cause of the release of the ammonia is also virtually impossible.
Further, in existing industrial ammonia refrigeration systems, ammonia will often remain in the common collection header after the release event has ended. The residual ammonia in the collection header can create subsequent false alarms triggered by the sniffer and create a hazard to personnel that might come into contact with the outlet of the common collection header even after the ammonia release event has ended.
Importantly, for each ammonia release event, the operator of the refrigeration system must account for the amount of ammonia released as a result of the ammonia release event in order to comply with current federal and industry regulatory compliance standards. In existing industrial ammonia refrigeration systems, measuring the amount of ammonia released is at best an inaccurate estimate. Because industrial ammonia refrigeration systems use standard release valves, determining the flow through a standard release valve during the ammonia release event is either unknown or poorly characterized. Further, determining the duration of the ammonia release event can be inaccurate because of the inaccuracy of the sniffers. While specialized release valves may be employed to more accurately determine the duration of an ammonia release event, the added cost makes such a solution unattractive and does not address the question of the flow rate through such release valves during the ammonia release event.